A {100}<001> texture has a stable recrystallization orientation of relatively high purity copper, which is texture orientation that is relatively easy to develop. When copper is rolled so as to be recrystallized, cube orientation is obtained, in which the <100> orientation is aligned with the rolling direction (RD), the normal direction (ND), and the transverse direction (TD) which is orthogonal to RD and ND.
Although it is not easy to stably form a high-density cube texture, it is attempted that a high-density cube texture is industrially utilized for flexible circuit board copper foil (Patent Literature 1 and 2) and rectangular copper wire for solar cell connection (Patent Literature 3 and 4) in recent years. Such cube texture has been realized in practice in some cases. A cube texture is formed on a material, in order to improve fatigue characteristics (Patent Literature 1 and 2) and reduce Young's modulus (Patent Literature 3) and yield strength (Patent Literature 4) for softening.
In general, strain is repeatedly loaded on a metal material that constitutes such flexible circuit board copper foil or rectangular copper wire for solar cell connection. For a flexible circuit board, strain is caused by bending at a hinge portion, a slide portion, or a folding portion of a mobile phone. For a solar cell conductor, heat strain is caused by a difference in the thermal expansion coefficient between silicon and copper.
For both purposes, a case of a fatigue failure of copper that is a metal material has been reported, which would be solved by increasing fatigue strength of copper material. Patent Literature 2 teaches to utilize the fact that a cube texture is excellent in terms of the improvement of fatigue characteristics of copper foil to which a high degree of bending with a small radius of curvature is applied when, for example, copper foil is incorporated into a thin device such as a mobile phone for use. Patent Literature 2 also teaches that anisotropy of mechanical characteristics of such cube texture was utilized to create an idea of a pattern in which the stress direction is aligned with the orientation of high breaking elongation.
In order to improve fatigue characteristics of metal material, it is common to improve metal strength and enhance breaking elongation. For such a purpose, it is common to refine crystal grains. In this regard, although coarsening of grain sizes through cube texture formation is contrary to the above from the viewpoint of material texture, for example, Patent Literature 2 proves that fatigue characteristics may improve by coarsening of crystal grains. It is therefore predicted that when a copper material may have a texture including an advanced cube texture and further have enhanced strength or breaking elongation, the copper material may have enhanced excellent fatigue characteristics.
It is difficult to improve strength and breaking elongation of a material having a highly developed cube texture. As stated above, a cube texture is usually formed by using a stable recrystallization orientation of relatively high-purity copper. This suggests that the number of dislocations or grain boundaries that originally act to increase strength is small. In addition, when it is tried to improve strength by the action of enhancing solid solution or promoting precipitation of alloy elements via alloying, changes in stacking-fault energy might cause the stable recrystallization orientation to vary or precipitates to inhibit grain growth, thereby inhibiting cube texture formation. For such reasons, a copper material having an advanced cube texture is limited in terms of types or concentrations of alloy elements added. In particular, no copper alloy with a highly advanced cube orientation, which contains elements at high concentrations that would promote precipitation, has been found.
As an aside, since a copper material having a developed cube texture is produced by rolling and recrystallization upon industrial production, the <100> main orientation may be in the rolling direction. In other words, the longitudinal direction of a rectangular copper wire for solar cell connection corresponds to the <100> orientation and the stress direction of a flexible circuit board corresponds to the <100> orientation without specific intention. However, when stress is applied in the <100> orientation, the orientation results in the smallest breaking elongation, among possible orientations. Therefore, a material having an advanced cube texture is excellent in terms of fatigue characteristics while the orientation which is most likely to be used (i.e., <100>) is undesirable in terms of fatigue characteristics. For such reasons, it is desirable to improve strength and enhance breaking elongation when stress is applied to a copper material having a highly developed cube texture in the <100> direction.